This invention concerns a tapping method and the relative device for electric arc furnaces, ladle furnaces or tundishes.
The invention is applied in the siderurgical field to achieve a controlled discharge, from the bottom or from the side, of the liquid metal, such as steel or its alloys, contained in melting volumes and in particular in electric arc furnaces and in ladle furnaces or in tundishes.
The state of the art covers electric arc furnaces and ladle furnaces or tundishes, or more generally melting volumes, on the bottom of which there is a casting channel which, thanks to the appropriate interception devices, can be opened on command to allow the liquid metal to be tapped when the melting cycle is complete.
In the state of the art, these devices normally comprise a plug element whose function is to close the tapping channel; at the end part of the tapping channel there is a quantity of sand which separates the liquid metal from the surface of the plug element.
When tapping is carried out, the plug element is opened and the liquid steel begins to flow down from the furnace once the sand has completely come out from the tapping channel.
This kind of application is particularly used in furnaces where the tapping channel is located in an eccentric position with respect to the floor of the hearth.
Because of this position, problems have often been found in the functioning of the furnace, causing increases in costs and danger for the workers in the area around the tapping area.
A first disadvantage is that the liquid metal often impregnates the sand inside the tapping channel and solidifies there or adheres at least partially to the walls of the channel.
This makes it necessary to intervene manually to free the tapping channel.
More particularly, it may be necessary to use a jet of oxygen to melt the solid plug which is created on the walls of the tapping channel, and this procedure may cause grave risks to the safety of the workers.
Moreover, this jet of oxygen causes great and premature wear in all those parts affected by the jet, which causes problems of a practical nature during the tapping step and extra costs for the replacement and/or maintenance of those components subject to wear.
This kind of device is also used when tapping is carried out from the sides of the furnace.
In this case, in some applications known to the state of the art, a device is used to intercept the flow of liquid metal which consists of a mechanical translation device, located at the sides on the vertical walls of the hearth, to close the tapping channel.
The axis of the tapping channel is placed in a sub-horizontal position.
The mechanical device is not cooled and substantially consists of a plate with a hole for the liquid metal to pass through.
The translation of the device only occurs through the interception of the liquid metal, after which the furnace is rotated in the opposite direction by an angle sufficient to prevent contact between the liquid metal and the tapping device.
Even if the absence of sand prevents the above-mentioned disadvantage from occurring, this system of tapping also has considerable disadvantages, such as for example a high energy consumption, an increase in the times of the production cycle so as to allow the furnace to be rotated, and also a heavy wear of the components.
The prior art document GB-A-440.859 provides a furnace which serves to cast liquid metal at progressively reduced speeds inside already finished casting molds.
This document teaches that the tapping hole is closed by a metallic plug during the preparatory phase of the bath of liquid metal. The plug is then melted by means of an induction coil so that the metal can be tapped.
This document teaches to use a pre-constituted plug, it does not teach to use mechanised closing systems.
FR-A-1.527.380 and JP-A-63-063566 teach to conserve the nozzle for casting by maintaining a solid skin in contact with the inner walls of the furnace through which the casting nozzle passes.
An induction heating device is included to control the thickness of the skin and to keep hot the molten metal passing through.
The resultant device is only useful during the tapping step.
EP-A-0.234.572 is substantially identical in its teaching to FR-A-1.527.380 associated with a sliding valve. According to the teachings of EP-A-0.234.572, the solidification of metallic parts in connection with the sliding valve is an undesired effect and therefore these teachings are only useful during the tapping step.
Not one of the prior art documents we have now considered teaches, or leads to consider, how to provide for the closure of the tapping hole and the automatic management of this closure both at the end-of-tapping step and at the start-of-tapping step.